In a gas turbine engine, the turbine is of multi-stage construction. Each stage comprises a rotating multi-bladed rotor and a nonrotating multi-vane stator. The blades of each rotor are circumferentially distributed on a disk connected to the main shaft. The shaft is rotatably supported by bearings. The turbine drives the compressor by way of the main shaft in response to aerodynamic interaction of the rotor blades and stator vanes with the high-energy gases expelled from the combustor.
The shaft-supporting bearings are disposed in a bearing compartment. During engine operation, friction in the rolling bearings and the rotating runners of associated seals generates heat. To cool the bearings and seals, a system for injecting oil into the bearing compartment is provided.
To prevent the escape of oil from the bearing compartment in main shaft applications, a conventional practice is to install circumferential seals with under-cooled runners, such as that depicted in FIG. 1. The term "circumferential seal" as used herein is intended to encompass a generic type of sealing device which consists primarily of a plurality of arcuate carbon material segments arranged circumferentially in abutting relationship to form a continuous ring which opposes a corresponding circumferential surface of a runner, forming a seal or rubbing interface therebetween.
In accordance with the conventional circumferential sealing arrangement depicted in FIG. 1, two segmented non-rotating carbon rings 2 and 2' are secured to the seal housing structure 4. Carbon rings 2 and 2' are urged into contact with the outer circumferential surface of the runner 6 by respective garter springs 8 and 8'. The runner 6 is connected to the main shaft 10 and rotates therewith.
Each garter spring encircles the outer diameter of the carbon segments of the respective sealing ring and provides a radially inwardly directed force. The carbon segment ends contain overlapping tongue and socket joints (not shown) to restrict leakage at the end gaps.
In accordance with this conventional arrangement, a windback seal 12 is machined into the seal housing structure 4. The windback seal generally consists of an open helical thread through which small quantities of oil may be discouraged from collecting at a sealing interface due to the "windage" generated by the outer circumferential surface of the rotating runner.
Circumferential seals are used to separate ambient areas of high pressure air (P.sub.a) from an oil wetted area at lower pressures (P.sub.s) and serve two major functions. First, circumferential seals prevent the leakage of oil from the lower pressure (P.sub.s) bearing compartment (i.e., oil sump 14). Second, circumferential seals minimize the flow rate of the hot air from the high pressure area to the oil wetted sump 14.
The conventional circumferential seal arrangement shown in FIG. 1 is a tandem air-air and air-oil seal design (two carbon rings) with an air pressurization system 16. Two carbon rings are used because the oil sump pressure is higher than the ambient pressure (P.sub.s &gt; P.sub.a) in this application. Pressurized air having a pressure higher than P.sub.s is supplied to the space between the carbon rings 2, 2' to ensure a positive air-to-oil sealing, that is, the pressure differential is such that air tends to flow into, not out of, the oil sump across any gap in the carbon seal.
For sealing applications where the internal engine ambient pressure is higher than the oil sump pressure (P.sub.a &gt; P.sub.s), the one carbon ring seal design shown in FIG. 2 can be used.
In these respective embodiments, the pressure drop between the high and low pressure ambients is taken across the stationary carbon ring 2 or rings 2 and 2'. A rubbing force is generated at the interface 18 of each carbon ring and the rotating runner 6. The frictional forces at the interface 18 generate significant amounts of heat. This heat must be either absorbed by the seal housing 4 and runner 6 or otherwise removed from the seal region.
Without adequate heat extraction, heating of the rubbing interface can become excessive, especially when the sealing pressure drops and rubbing speeds are high. An excessively high temperature at the interface can cause severe damage to the seal. In particular, oil coke can fill up the seal bore reliefs, recess grooves and end gaps of the carbon ring. In addition, excessive wear of the carbon segments and cracking and grooving of the runner coating and surface can occur. It can also result in excessive oil leakage through the seal and correspondingly high oil consumption.
The conventional technique to avoid over-heating in the vicinity of the seal interface is to cool that vicinity with oil. In particular, it is advantageous to inject cooling oil under the runner. When an under-cooled runner is used (see FIG. 1), an oil injector 20 is provided with two outlets 22 and 24. The oil jet 27 exiting the injector via outlet 24 cools the bearing, generally indicated by the numeral 26 in FIG. 1. The oil jet 28 exiting the injector via outlet 22 is directed toward the peripheral edge of the runner. The oil jet 28 is scrolled to the inner circumferential surface 30 of runner 6 to remove the heat conducted from the rubbing interface 18 to the outer circumferential surface of the runner.
The thorniest problem associated with this under-cooled design is the thin film depth and sluggish velocity of the oil on the inner circumferential surface 30 of the runner 6. In most engine lubricating systems, due to factors such as space limitation and windage deflection of the oil jet, the oil jetting under the runner cannot be made to impinge directly under the rubbing interface with high wiping velocity. Therefore, heat absorption by the sluggishly moving oil film must occur by very slow molecular diffusion. The resulting heat transfer coefficient is very low.
In addition, it is likely that the oil jet directed toward the runner in accordance with the conventional sealing arrangement will be partially deflected by the runner/shaft windage. The windage stream also causes oil jet fanning. The deflected and/or fanning oil jet can easily miss the runner annulus and scroll into the windback seal, resulting in excess oil ingestion at the seal. Oil shearing between the carbon bore and the outer circumferential surface of the runner will generate significantly more heat than that generated by dry frictional rubbing. Therefore, the temperature of the rubbing interface 18 will become much higher.